1. Field of Invention
The present invention relates generally to the field of wireless transceiver systems and, more particularly, to electronic circuits, such as mixers, used in wireless transceiver systems.
2. Discussion of Related Art
A mixer is a device that performs frequency translation by mixing two signals (and possibly their harmonics). The process of mixing the two signals leads to down-conversion or up-conversion of the input frequency to a different output frequency. Up-conversion mixers are used in transmitters, and down-conversion mixers are used in receivers. There are two types of down-conversion mixers: direct and low-IF.
Referring to FIG. 1, a mixer 100 can be described as a three port device, comprising a radio frequency (RF) port 102, an intermediate frequency (IF) port 104, and a local oscillator (LO) port 106. For an up-conversion mixer, the RF port 102 is where the desired input signal is sensed, the LO port 106 senses the signal that is used to down-convert the input signal, and the down-converted signal results at the IF port 104. For an up-conversion mixer, the RF and IF ports 102, 104 would be reversed, and the LO port 106 would sense the signal used to up-convert the input signal.
Modulation is the process of varying a periodic waveform, i.e., a tone, in order to use that signal to convey a message. A mixer can act as an amplitude modulator or a demodulator. The process of modulation as performed by the mixer is the same as that of mixing, except that the amplitude modulation is often used to impose the amplitude of a lower frequency signal on top of a high frequency signal. The modulation process up-converts the input signal to a so-called “carrier frequency” and transmits it out. The demodulation process does the opposite. That is, the process of demodulation down-converts the carrier frequency signal and obtains the information/message from the resulting lower frequency signal.
In some instances, both the upper and lower sidebands of a modulated carrier signal will contain information. This double sideband modulation occurs when both the upper and lower sideband modulation appear at the output of the mixer. For down-conversion, the upper and lower sideband frequencies are given by:fRF+fLO and fRF−fLO  (1)and for up-conversion, the upper and lower sideband frequencies are given by:fIF+fLO and fIF−fLO  (2)
Due to device non-linearities, the output spectrum of the two mixed signals is usually not only composed of the sum and difference frequencies, but includes various other frequencies as well. These other frequencies are generally much lower in power compared to the fundamental output frequencies given by equations (1) and (2). In general, the output of a down-conversion mixer will be composed of the following frequencies:m×fRF±n×fLO  (3)and the output of an up-conversion mixer will be composed of the following frequencies:m×fIF±n×fLO  (4)where m and n can be any positive or negative integer value.
The mixer discussed above can also be referred as a harmonic mixer, because it takes the fundamental harmonic frequency of the local oscillator and mixes it with the fundamental IF or the RF frequency of the input signal. Sometimes for direct-digital or zero-IF operations, in order to eliminate the unwanted effects of LO self-mixing which occurs in many fundamental frequency mixers, a so-called sub-harmonic mixer is used. In the sub-harmonic mixer, the output signal includes the sum and difference of the fundamental and one of the harmonics of the two input signals, for example, as given by:f1+2f2 and f1−2f2  (5)
One important decision in the design of an RF front-end module for a transceiver is the type of up-conversion or down-conversion mixer that will be implemented. There are two primary types of mixers, namely active and passive mixers. Both types have benefits and drawbacks. At a high level, a passive mixer topology is usually selected when linearity and low noise are important design factors. Passive mixers are also often used in zero-IF receiver architectures due to the superior 1/f noise (also referred to as “flicker noise”) and DC offset performance of the passive topology. 1/f noise is so-called due to its inverse relationship to frequency; that is, as the frequency increases, the 1/f noise decreases. An active mixer topology is generally selected when conversion gain is required in the front-end module, such as in receivers with a high second stage noise contribution.